The invention relates to a multilayer mirror, in particular a dispersive multilayer mirror, comprising several individual layers of different properties applied to a substrate and following each other via parallel, plane interfaces, and a plane front face on the side of beam impact.
In laser technology, shorter laser pulses comprising pulse durations in the picosecond and femtosecond range (to less than 10 femtoseconds) are increasingly used. Apart from their use in scientific fields, such short pulse laser arrangements increasingly are utilized also e.g. for the processing of materials with the highest precision, in ultrarapid spectroscopy, in the optical broadband communication and in femtochemistry. The laser crystals employed in such short pulse laser arrangements, cf. e.g. WO 98/10494 A; have excellent thermal properties as well as broad fluorescence bands so as to enable the generation of laser pulses having pulse durations of less than 10 or even less than 5 femtoseconds. Here, in particular, laser crystals are used which are doped with transition metals, such as, particularly, the titan sapphire (TI:S) laser crystal.
One problem in the generation of such ultra-short laser pulses resides in the remaining optical components of the respective laser system, wherein it would be particularly important to have wide-band dispersive components available.
It has already been suggested to provide dispersive mirrors for laser arrangements in thin-layer technique, cf., e.g., U.S. Pat. No. 5,734,503 A. In doing so, the mirrors are comprised of a plurality of individual layers having different properties, i.e. having respectively alternating higher and lower refraction indices, which, when reflecting an ultra-short laser pulse—which has a correspondingly large bandwidth in the frequency range fulfill their function: the different wave length components of the laser beam enter to different depths into the individual layers of the mirror before being reflected. In this manner, the different frequency components are delayed for different amounts of time, corresponding to the respective depth of entry; if a negative group delay dispersion must be attained, the short-wave wave packs will be reflected more outwardly; the long-wave components, however, will enter more deeply into the mirror before they are reflected. This means that the long-wave frequency components will be temporally delayed relative to the short-wave components. In this manner, a dispersion compensation can be attained for a short-pulse laser beam in a laser arrangement. Pulses of a particularly short time range have a wide frequency spectrum, with the different frequency components of the laser beam, primarily in a dense propagation medium (such as, e.g., in a laser crystal), or also in air, however, “seeing” a different refraction index (i.e., the optical thickness of the propagation medium is differently large for the different frequency components of the laser pulses); the different frequency components of the laser pulse therefore will be differently delayed when passing through the propagation medium. This effect is to be counteracted by the above-mentioned dispersion compensation at the known dispersive thin film laser mirrors, at which a negative group delay dispersion (GDD) is effected. These known mirrors are also termed “chirped mirrors” (CM), and constitute a substantial progress as compared to the previously used delaying elements comprising prisms. It has been possible for the first time to obtain laser pulses having pulse durations of 10 femtoseconds and below directly from a laser oscillator, and the laser systems have become more compact and reliable. The CM mirrors control the wave length dependence of the group delay as mentioned by the depth of entry of the various spectral components in the multilayer structure.
One problem with these CM mirrors and, quite generally, with comparable multilayer mirrors consists in that a reflection which is largely independent of the wave length (e.g. in the order of 3%) will occur at the interface between the uppermost layer and the environment, i.e. on the front face, where the radiation will impinge. As a consequence, interferences between beams reflected at this front face, and beams reflected more deeply in the multilayer structure of the mirror will occur, these interference effects possibly causing a distortion of the reflectivity and, above all, to a pronounced distortion of the phase characteristic of the mirror. To counteract this effect at least partially, it has already been suggested (cf. F. X. Kärntner, N. Matuscheck, T. Tschibili, U. Keller, H. A. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch, T. Tschudi, “Design and fabrication of double-chirped mirrors”, 1997, Opt. Lett 22, 831; and G. Tempea, F. Krausz, Ch. Spielmann, K. Ferencz, “Dispersion control over 150 THz with chirped dielectric mirrors” 1998, IEEE JSTQE 4, 193, respectively), to provide an anti-reflecting coating or a narrow-band barrier filter at the front face, i.e. at the interface with the environment (air, as a rule). To effectively suppress interfering resonances, the reflection at the front face should be in the order of merely 10−4. Antireflection layers and barrier filters may, however, approximate such a property merely over a very limited band width. Accordingly, in the past, with 800 nm radiation, dispersive mirrors could only be operated over band widths of 150-160 THz. Moreover, a complete suppression of the resonance interference effects is not even possible over such a band width, and the dispersion curve often exhibits pronounced fluctuations.